In digital computing apparatus and related equipment it is often desirable, for maintenance purposes, to be able to manually enter a binary number pattern into a register and to have an indication of the state of each stage of the register from a location which may be remote from the register in question. For example, in many computing systems, the operator's console or the maintenance unit may be located several feet from the central processor unit (CPU) and for convenience, it is desirable to be able to enter a binary data word or an address into a register in the CPU from the remote console. In the past this has been accomplished by having a push-button switch and an associated indicator lamp connected to each stage of the CPU's register by wires such that when the push-button switch is operated, the binary state of the register stage can be changed and a lamp illuminated or extinguished to indicate the condition of the stage.
Problems have arisen in such prior art arrangements because of the inherent contact bounce or vibration present in most all mechanically operated switches which are commercially available at a reasonable price. With these switches, each operation produces a series of makes and breaks rather than a single make or break due to contact bounce. As such, the register stage to which it is connected may be toggled an indeterminate number of times and end up in a state different from that which was intended by the operator.
Prior art solutions to the contact bounce problem have involved the use of delay circuits triggered by the first pulse of a series of noise pulses which disable the logic network for a period sufficient to enable the noise to subside. Exemplary of this approach is the Morgan et al U.S. Pat. No. 3,795,823 and the McIntosh U.S. Pat. No. 3,673,434. Another proposed solution as set forth in the Brahan U.S. Pat. No. 3,381,143 involves the use of a set-reset type flip-flop interposed between the switch contacts and the output logic circuit such that when the mechanical switch is repositioned, the flip-flop switches state, but successive makes and breaks caused by bounce do not reverse the state of the flip-flop and therefore do not affect the voltage appearing at the output of the flip-flop.
The present invention is considered to be an improvement over that disclosed in the aforementioned Brahan patent in that an inexpensive double-pole double-throw push-button switch and indicator assembly can be made to operate as a toggle switch having a debounce feature, the power supply for the indicator being the same as that for the toggle and debounce circuitry.
In the apparatus of the present invention a pair of transition edge-triggered integrated circuit flip-flops are interconnected with each other and with the push-button switch and indicator lamp so that a first depression of the push-button switch will cause the state of one of the flip-flops to switch and operate the indicator and a second depression of the switch will cause the flip-flop to revert to its prior state and extinguish the indicator. The other flip-flop prevents noise produced by contact bounce from affecting the state of said one flip-flop.